Abstract: Gillespie (1974) demonstrated mathematically that reducing the variance in offspring number could increase a genotype's fitness above that of another genotype having the same mean reproductive output. The implication was that natural selection could act upon the variance, as well as the mean of reproductive success, increased variance contributing negatively to fitness. Slatkin (1974) recognized the applicability of Gillespie's argument to the evolution of life histories and encouraged population ecologists to apply this idea to life history studies. Nichols et al. (1976) independently made a similar plea, suggesting that rather than represent the reproductive effort of a population as a point along an r-K continuum, it would be more appropriate to define effort as a frequency distribution. Consistent with this idea, Ekbohm et al. (1980) and Real (1980a, 1980b) added that an organism's fitness is likely to result from a trade-off between the mean and variance; many combinations of means and variances in fitness components can produce the same overall fitness.